Various MEMS devices are becoming increasingly popular. MEMS transducers, including MEMS capacitive sensors, such as microphones, and capacitive output transducers, such as speakers are increasingly being used in portable electronic devices such as mobile telephones and portable computing devices.
Transducer devices formed using MEMS fabrication processes typically comprise one or more membranes with electrodes for read-out/drive deposited on the membranes and/or a substrate. In the case of MEMS pressure sensors and microphones, for example, the read out is usually accomplished by measuring the capacitance between a pair of electrodes which will vary as the distance between the electrodes changes in response to sound waves incident on the membrane surface.
FIGS. 1a and 1b show a schematic diagram and a perspective view, respectively, of a known capacitive MEMS microphone device 100. The capacitive microphone device 100 comprises a membrane layer 101 which forms a flexible membrane which is free to move in response to pressure differences generated by sound waves. A first electrode 102 is mechanically coupled to the flexible membrane, and together they form a first capacitive plate of the capacitive microphone device. A second electrode 103 is mechanically coupled to a generally rigid structural layer or back-plate 104, which together form a second capacitive plate of the capacitive microphone device. In the example shown in FIG. 1a the second electrode 103 is embedded within the back-plate structure 104.
In use, in response to a sound wave corresponding to a pressure wave incident on the microphone, the membrane is deformed slightly from its equilibrium position. The distance between the lower electrode 103 and the upper electrode 102 is correspondingly altered, giving rise to a change in capacitance between the two electrodes that is subsequently detected by electronic circuitry (not shown).
The capacitive microphone is formed on a substrate 105, for example a silicon wafer which may have upper and lower oxide layers 106, 107 formed thereon. A cavity 108 in the substrate and in any overlying layers (hereinafter referred to as a substrate cavity) is provided below the membrane, and may be formed using a “back-etch” through the substrate 105. The substrate cavity 108 connects to a first cavity 109 located directly below the membrane. These cavities 108 and 109 may collectively provide an acoustic volume thus allowing movement of the membrane in response to an acoustic stimulus. Interposed between the first and second electrodes 102 and 103 is a second cavity 110.
The first cavity 109 may be formed using a first sacrificial layer during the fabrication process, i.e. using a material to define the first cavity which can subsequently be removed, and depositing the membrane layer 101 over the first sacrificial material. Formation of the first cavity 109 using a sacrificial layer means that the etching of the substrate cavity 108 does not play any part in defining the diameter of the membrane. Instead, the diameter of the membrane is defined by the diameter of the first cavity 109 (which in turn is defined by the diameter of the first sacrificial layer) in combination with the diameter of the second cavity 110 (which in turn may be defined by the diameter of a second sacrificial layer). The diameter of the first cavity 109 formed using the first sacrificial layer can be controlled more accurately than the diameter of a back-etch process performed using a wet-etch or a dry-etch. Etching the substrate cavity 108 will therefore define an opening in the surface of the substrate underlying the membrane 101.
The sacrificial material used to define the first and second cavities is dimensioned so as to provide a desired equilibrium separation between the membrane layer 101 and the substrate 105 and also between the membrane layer 101 and the back-plate 104 so as to provide good sensitivity and dynamic range in use. In normal operation the membrane may deform within the volume defined by the first and second cavities without contacting the back-plate and/or substrate 105.
A plurality of holes, hereinafter referred to as bleed holes 111, connect the first cavity 109 and the second cavity 110. The bleed holes allow the pressure in the first and second cavities to equalise over a relatively long timescales (in acoustic frequency terms) which reduces the effect of low frequency pressure variations, e.g. arising from temperature variations and the like, but without significantly impacting on sensitivity at the desired acoustic frequencies.
As mentioned, the membrane may be formed by depositing at least one membrane layer 101 over a first sacrificial material. In this way the material of the membrane layer(s) may extend into the supporting structure, i.e. the side walls, supporting the membrane. The membrane and back-plate layer may be formed from substantially the same material as one another, for instance both the membrane and back-plate may be formed by depositing silicon nitride layers. The membrane layer may be dimensioned to have the required elasticity whereas the back-plate may be deposited to be a thicker and therefore more rigid structure. Additionally various other material layers could be used in forming the back-plate 104 to control the properties thereof. The use of a silicon nitride material system is advantageous in many ways, although other materials may be used, for instance MEMS transducers using polysilicon membranes are known.
In some applications, the microphone may be arranged in use such that incident sound is received via the back-plate. In such instances a further plurality of holes, hereinafter referred to as acoustic holes 112, are arranged in the back-plate 104 so as to allow free movement of air molecules, such that the sound waves can enter the second cavity 110. The first and second cavities 109 and 110 in association with the substrate cavity 108 allow the membrane 101 to move in response to the sound waves entering via the acoustic holes 112 in the back-plate 104. In such instances the substrate cavity 108 is conventionally termed a “back volume”, and it may be substantially sealed.
In other applications, the microphone may be arranged so that sound may be received via the substrate cavity 108 in use. In such applications the back-plate 104 is typically still provided with a plurality of holes to allow air to freely move between the second cavity and a further volume above the back-plate that may be substantially sealed to provide a back volume. In such instances the substrate cavity 108 is conventionally termed a “front volume”.
It should also be noted that whilst FIG. 1 shows the back-plate 104 being supported on the opposite side of the membrane to the substrate 105, arrangements are known where the back-plate 104 is formed closest to the substrate with the membrane layer 101 supported above it.
One skilled in the art will appreciate that MEMS transducers are typically formed on a wafer before being singulated. Increasingly it is proposed that at least some electronic circuitry, e.g. for read-out and/or drive of the transducer, is also provided as part of an integrated circuit with the transducer. For example a MEMS microphone may be formed as an integrated circuit with at least some amplifier circuitry and/or some circuitry for biasing the microphone. The footprint of the area required for the transducer and any circuitry will determine how many devices can be formed on a given wafer and thus impact on the cost of the MEMS device. There is therefore a general desire to reduce the footprint required for fabrication of a MEMS device on a wafer.
A number of membrane designs have been proposed which more efficiently use a generally square or rectangular membrane area rather than a circular area. This beneficially results in a more efficient use of the silicon wafer during fabrication.
In addition, to be suitable for use in portable electronic devices, capacitive transducers should preferably be able to survive the expected handling and use of the portable device, which may include the device being accidentally dropped. If a device such as a mobile telephone is subject to a fall, this can result not only in a mechanical shock due to impact but also a high pressure impulse incident on a MEMS transducer. For example, a mobile telephone may have a sound port for a MEMS microphone on one face of the device. If the device falls onto that face, some air may be compressed by the falling device and forced into the sound port. This may result in a high pressure impulse incident on the transducer. It has been found that in conventional MEMS transducers high pressure impulses can potentially lead to damage of the transducer.
Furthermore, stresses can arise in the membrane layer even when at equilibrium e.g. due to thermal and chemical effects during deposition in manufacture. A certain amount of stress is beneficial in enhancing the elasticity of the membrane and hence the microphone mechanical sensitivity. However additional stresses may occur across the membrane or in particular regions, for example due to the way in which the membrane is supported in a fixed relation relative to the substrate, which can also result in membrane failure or damage. MEMS transducers according to the present invention are intended to respond to the acoustic pressure waves which manifest as transient stress waves on the membrane surface which also provide additional components of stress especially during high pressure impulses. Thus, it will be appreciated that any stresses exhibited within a membrane layer when at equilibrium may potentially have a detrimental impact on the performance or robustness of a transducer.